A passive optical network (PON) system has been employed for an optical access network including a fiber to the home (FTTH). In the PON system, a point-to-multipoint structure is provided from an optical line terminal (OLT) to a plurality of optical network terminals (ONTs). Communication from the OLT to the ONT is called downlink communication. Information is distributed through broadcast. Only information for each user is passed through a corresponding ONT. In contrast, communication from the ONT to the OLT is called uplink communication. A transmission enable signal called a grant is transmitted from the OLT to the ONT, and information may be transmitted to the OLT from an ONT designated by the transmission enable signal.
There is provided PON protection as high-reliability control in the PON system. The PON protection is achieved using a duplexed transmission line in a PON section. The PON protection may be useful for business use or multicast communication for which high reliability is required. In particular, though the PON protection requires high reliability in software distribution and information broadcast, the PON protection is required for improving the reliability on a transmission level in an application in which transmission may not be confirmed because of communication through multicast.
With respect to the PON protection, PON systems which are described in Patent Documents 1 to 3 and specified in ITU-T G.983.1 and the recommendation of ITU-T G.983.5 which is described in Non-patent Document 1 are provided as conventional technologies.
According to Patent Documents 1 to 3, protection control using an operation administration and maintenance (OAM) cell as a physical layer is performed in an ATM-PON which is an ATM-based PON or a B-PON, which is specified in ITU-T G.983.1. A PON topology is a point-to-multipoint structure. According to the systems, path switch information called K1 and K2 specified in ITU-T G.783 are mapped on the OAM cell as the physical layer. Therefore, even in the PON topology, the same control as point-to-point determined in ITU-T G.783 may be realized.
In ITU-T G.983.5 described in Non-patent Document 1, a protection system in the PON system specified in ITU-T G.983.1 is specified. In this case, information specified in ITU-T G.783 is transferred onto the PON by using the OAM cell to realize switching from a current transmission line to an auxiliary transmission line.
In ITU-T G.983.5, there are a tree switch and a branch switch. A condition of the former is that all ONTs are made redundant. At the time of occurrence of fault, simultaneous switching is performed by an OLT-side interface. On the other hand, in the latter, an environment in which an ONT which is made redundant and an ONT which is not made redundant are mixed is assumed. In this case, both OLT-side interfaces operate. Therefore, switching is performed only when a fault occurs in an ONT-side interface. In ITU-T G.983.5, the ATM is assumed as transmission means. Thus, in the case of the tree switch, an OAM at a physical interface level detects a fault and controls switching. In the case of the branch switch, an OAM at a VP level of an ATM layer is used.    Patent Document 1: JP 2002-152135 A    Patent Document 2: JP 2002-57679 A    Patent Document 3: WO 2002/049290    Non-patent Document 1: ITU-T G.983.5(01/2002)